A reactive distillation process for producing a dialkyl carbonate and a diol through reaction between a cyclic carbonate and an aliphatic monohydric alcohol was first disclosed by the present inventors (see Patent Document 1: Japanese Patent Application Laid-Open No. 4-198141, Patent Document 2: Japanese Patent Application Laid-Open No. 4-230243, Patent Document 3: Japanese Patent Application Laid-Open No. 9-176061, Patent Document 4: Japanese Patent Application Laid-Open No. 9-183744, Patent Document 5: Japanese Patent Application Laid-Open No. 9-194435, Patent Document 6: International Publication No. WO97/23445 (corresponding to European Patent No. 0889025, and U.S. Pat. No. 5,847,189), Patent Document 7: International Publication No. WO99/64382 (corresponding to European Patent No. 1086940, and U.S. Pat. No. 6,346,638), Patent Document 8: International Publication No. WO00/51954 (corresponding to European Patent No. 1174406, and U.S. Pat. No. 6,479,689), Patent Document 9: Japanese Patent Application Laid-Open No. 2002-308804, Patent Document 10: Japanese Patent Application Laid-Open No. 2004-131394), and patent applications in which such a reactive distillation system is used have subsequently also been filed by other companies (see Patent Document 11: Japanese Patent Application Laid-Open No. 5-213830 (corresponding to European Patent No. 0530615, and U.S. Pat. No. 5,231,212), Patent Document 12: Japanese Patent Application Laid-Open No. 6-9507 (corresponding to European Patent No. 0569812, and U.S. Pat. No. 5,359,118), Patent Document 13: Japanese Patent Application Laid-Open No. 2003-119168 (corresponding to International Publication No. WO03/006418), Patent Document 14: Japanese Patent Application Laid-Open No. 2003-300936, Patent Document 15: Japanese Patent Application Laid-Open No. 2003-342209). In the case of using the reactive distillation system for this reaction, the reaction can be made to proceed with a high conversion. However, reactive distillation processes proposed hitherto have related to producing the dialkyl carbonate and the diol either in small amounts or for a short period of time, and have not related to carrying out the production on an industrial scale stably for a prolonged period of time. That is, these processes have not attained the object of producing a diol continuously in a large amount (e.g. not less than 1 ton/hr) stably for a prolonged period of time (e.g. not less than 1000 hours, preferably not less than 3000 hours, more preferably not less than 5000 hours).
For example, the maximum values of the height (H: cm), diameter (D: cm), and number of stages (n) of the reactive distillation column, the amount produced P (kg/hr) of ethylene glycol, and the continuous production time T (hr) in examples disclosed for the production of dimethyl carbonate (DMC) and ethylene glycol (EG) from ethylene carbonate and methanol are as in Table 1.
TABLE 1PATENTDOCUMENTH: cmD: cmNO. STAGES: nP: kg/hrT: hr11002300.07340041605400.213NOTE 551605400.358NOTE 572004PACKING COLUMN (Dixon)0.528NOTE 58NOTE 15600.140NOTE 59NOTE 15600.161NOTE 510NOTE 15600.161NOTE 5112503PACKING COLUMN (Raschig)0.154NOTE 512NOTE 2NOTE 2NOTE 20.256NOTE 513NOTE 3NOTE 342NOTE 4NOTE 514NOTE 3NOTE 3302490NOTE 51520015 PACKING COLUMN (BX)19NOTE 5NOTE 1: OLDERSHAW DISTILLATION COLUMN.NOTE 2: NO DESCRIPTION WHATSOEVER DEFINING DISTILLATION COLUMN.NOTE 3: ONLY DESCRIPTION DEFINING DISTILLATION COLUMN IS NUMBER OF STAGES.NOTE 4: NO DESCRIPTION WHATSOEVER OF PRODUCED AMOUNT.NOTE 5: NO DESCRIPTION WHATSOEVER REGARDING STABLE PRODUCTION FOR PROLONGED PERIOD OF TIME.
In Patent Document 14 (Japanese Patent Application Laid-Open No. 2003-300936), it is stated at paragraph 0060 that “The present example uses the same process flow as for the preferred mode shown in FIG. 1 described above, and was carried out with the object of operating a commercial scale apparatus for producing dimethyl carbonate and ethylene glycol through transesterification by a catalytic conversion reaction between ethylene carbonate and methanol. Note that the following numerical values in the present example can be adequately used in the operation of an actual apparatus”, and as that example it is stated that 3750 kg/hr of dimethyl carbonate and 2490 kg/hr of ethylene glycol were specifically produced. The scale described in that example corresponds to an annual production of 30,000 or more tons of dimethyl carbonate, and hence this implies that operation of the world's largest scale commercial plant using this process had been carried out at the time of the filing of the patent application for Patent Document 14 (Japanese Patent Application Laid-Open No. 2003-300936) (Apr. 9, 2002). However, even at the time of filing the present application, there is not the above fact at all. Moreover, in the example of Patent Document 14 (Japanese Patent Application Laid-Open No. 2003-300936), exactly the same value as the theoretically calculated value is stated for the amount of dimethyl carbonate produced, but the yield for ethylene glycol is approximately 85.6%, and the selectivity is approximately 88.4%, and hence it cannot really be said that a high yield and high selectivity have been attained. In particular, the low selectivity indicates that this process has a fatal drawback as an industrial production process. (Note also that Patent Document 14 (Japanese Patent Application Laid-Open No. 2003-300936) was deemed to have been withdrawn on Jul. 26, 2005 due to examination not having been requested).
With such a reactive distillation process, there are very many causes of fluctuation such as composition variation due to reaction and composition variation due to distillation in the distillation column, and temperature variation and pressure variation in the column, and hence continuing stable operation for a prolonged period of time is often accompanied by difficulties, and in particular these difficulties are further increased in the case of handling large amounts. To continue mass production of a dialkyl carbonate and a diol using the reactive distillation process stably for a prolonged period of time while maintaining high yield and high selectivity, and thus produce a high-purity diol, the process must be cleverly devised. However, the only description of continuous stable production for the prolonged period of time with the reactive distillation process proposed hitherto has been the 200 to 400 hours in Patent Document 1 (Japanese Patent Application Laid-Open No. 4-198141) and Patent Document 2 (Japanese Patent Application Laid-Open No. 4-230243).
The present inventors have proposed an industrial reactive distillation process that enables a dialkyl carbonate and a diol to be mass-produced continuously and stably for a prolonged period of time with high yield and high selectivity, but in addition to this, a process enabling a high-purity diol to be separated out and purified in a large amount stably for a prolonged period of time from a high boiling point reaction mixture continuously withdrawn in a large amount from a lower portion of the reactive distillation column is also required, a process for producing a large amount of a high-purity diol with a high yield having been called for. The present invention has been devised to attain this object.
As shown in Table 1, with the exception of Patent Document 14 (Japanese Patent Application Laid-Open No. 2003-300936), the amount of the diol produced per hour using the reactive distillation processes proposed hitherto has been a small amount. Moreover, with the process of Patent Document 14 (Japanese Patent Application Laid-Open No. 2003-300936), it is stated that approximately 2490 kg/hr of ethylene glycol containing approximately 130 kg/hr of unreacted ethylene carbonate and approximately 226 kg/hr of dihydroxyethyl carbonate was obtained as a column bottom component from a fourth step distillation column. However, this is merely a statement of the composition of the reaction mixture, there being no description whatsoever of production of a high-purity diol.
As a process for producing a diol of relatively high purity using reactive distillation and a diol purifying column, a process is known in which the diol is obtained from a side cut of the diol purifying column. For example, in the example (FIG. 5) in Patent Document 12 (Japanese Patent Application Laid-Open No. 6-9507 (corresponding to European Patent No. 0569812, and U.S. Pat. No. 5,359,118)), a high boiling point reaction mixture withdrawn from a lower portion of a reactive distillation column is fed into a thin film evaporator (III), high boiling point matter obtained therefrom is fed into a thin film evaporator (IV), low boiling point evaporated matter obtained therefrom is fed into a distillation column (VII), and ethylene glycol is obtained as a side cut component 22 from an enrichment section of the distillation column (VII), and then purification is further carried out using a purifier (IX), whereby high-purity ethylene glycol is produced in an amount of 255 g/hr. That is, in the process of Patent Document 12 (Japanese Patent Application Laid-Open No. 6-9507 (corresponding to European Patent No. 0569812, and U.S. Pat. No. 5,359,118)), high-purity ethylene glycol is not obtained from the high boiling point reaction mixture until four purifying apparatuses have been used. Furthermore, the process of Patent Document 12 (Japanese Patent Application Laid-Open No. 6-9507 (corresponding to European Patent No. 0569812, and U.S. Pat. No. 5,359,118)) is a process in which a small amount of ethylene glycol is produced, there being no suggestions whatsoever regarding a process for producing a large amount (e.g. not less than 1 ton/hr) of a diol stably for a prolonged period of time (e.g. not less than 5000 hours).
Moreover, in, for example, example 1 (FIG. 5) in Patent Document 15 (Japanese Patent Application Laid-Open No. 2003-342209), a high boiling point reaction mixture withdrawn from a lower portion of a reactive distillation column is fed into a second distillation column 4, high boiling point matter obtained therefrom is fed into a hydrolysis reactor 7, the reaction mixture therefrom is fed into a decarboxylation tank (gas-liquid separator 8), a liquid component obtained therefrom is fed into a third distillation column 10, and ethylene glycol is produced in an amount of 19 kg/hr as a side cut component from a stripping section of the third distillation column 10. However, with the process of Patent Document 15 (Japanese Patent Application Laid-Open No. 2003-342209), the ethylene glycol obtained contains 0.2% by weight of diethylene glycol. To obtain ethylene glycol of a high purity as required as a starting material for a PET fiber or a PET resin using the process of Patent Document 15 (Japanese Patent Application Laid-Open No. 2003-342209), at least one further purifying apparatus is thus required. That is, with the process of Patent Document 15 (Japanese Patent Application Laid-Open No. 2003-342209), ethylene glycol is obtained from a side cut outlet installed in the stripping section, which is below an inlet for feeding into the distillation column, but the purity of the ethylene glycol is insufficient, and moreover the process of Patent Document 15 (Japanese Patent Application Laid-Open No. 2003-342209) is a process in which a small amount of ethylene glycol is produced, there being no suggestions whatsoever regarding a process for producing a large amount (e.g. not less than 1 ton/hr) of a diol stably for a prolonged period of time (e.g. not less than 5000 hours).
Moreover, in, for example, example 10 (FIG. 6) in Patent Document 8 (International Publication No. WO00/51954 (corresponding to European Patent No. 1174406, and U.S. Pat. No. 6,479,689)) and example 1 (FIG. 1) in Patent Document 9 (Japanese Patent Application Laid-Open No. 2002-308804), high-purity ethylene glycol is obtained from a side cut outlet installed in an enrichment section of an EG purifying column 41, which is above an inlet for feeding into the column, but in each case the amount produced is a small amount of less than 200 g/hr, there being no suggestions whatsoever regarding a process for producing a large amount (e.g. not less than 1 ton/hr) of a diol stably for a prolonged period of time (e.g. not less than 5000 hours).
Approximately 16 million tons per year (2004) of ethylene glycol is produced worldwide, but hitherto all of this has been through a hydration method in which water is added to ethylene oxide. However, as shown by the statement “Production of EG (ethylene glycol) is by a hydration reaction of EO (ethylene oxide), the reaction generally being carried out . . . at 150 to 200° C. At this time, not only is the target substance MEG (monoethylene glycol) produced, but moreover DEG (diethylene glycol) and TEG (triethylene glycol) are also by-produced. The proportions of these products depend on the water/EO ratio, and to obtain MEG with a selectivity of approximately 90%, the water/EO ratio must be made to be approximately 20 as a molar ratio. A large amount of water must thus be distilled off in an EG purification step, and a large amount of thermal energy is consumed in this. . . . With regard to synthesis of EG from EO, it is not an overstatement to say that this is an imperfect process from the viewpoint of energy efficiency.” in Non-Patent Document 1 (Japan Petroleum Institute (ed.), “Sekiyu-kagaku Purosesu” (“Petrochemical Processes”), pages 120 to 125, Kodansha, 2001), this industrial production process (conventional process) has great drawbacks both from the perspective of the ethylene glycol yield and selectivity, and the perspective of energy saving.